Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
The simplest procedure for incorporating an active agent into a porous material, such as wood, is to soak the porous material in a bath containing a solution in which is dissolved or suspended an active agent, allowing the solution to penetrate into the pores of the porous material, removing the soaked material from the solution and allowing it to dry. The solvent is usually selected so as to preferentially evaporate, leaving the active agent behind in the pores of the porous material.
This fairly simple process has been used to incorporate boron compounds (for example, boric acid) into wood for protection against borers. The wood to be treated is placed in an aqueous solution of the boron compounds until the solution has adequately soaked into the pores of the material. The process is fairly slow and depends on the nature of the wood and the cross-sectional area of the material which is to be impregnated. When the wood is believed to be sufficiently soaked through with the treating solution, it is removed from the bath and the solvent is allowed to drain off. The wood is then left to dry naturally before being used. The water content of such treated woods is usually very high when freshly treated.
In some cases alternate applications of vacuum and pressure are used to force the preservative solution into the wood. A vacuum may sometimes be used to remove excess fluid. However, the final drying step, in which the preservative binds to the wood, is invariably completed by natural drying because accelerated drying interferes with the binding process, degrades the product, or the capital equipment is too expensive for the time gained. This drying step is the rate determining step for the whole process.
One alternative impregnation process involves active ingredients dissolved in light organic solvents, usually referred to as LOSP (Light Organic Solvent Preservation) process. The LOSP process is widely used for the impregnation of wood with water-insoluble, organic active agents such as fungicides (for example copper naphthenate) and insecticides (such as synthetic pyrethroids).
The LOSP process has been used, for example, to incorporate copper naphthenate into radiata pine. Copper naphthenate is a fungicide available commercially as a 5% or 8% solution in liquid hydrocarbons. The copper naphthenate is diluted to a desired working strength by the addition of further quantities of hydrocarbon or white spirit, which act as a carrier. The radiata pine to be treated is placed in an autoclave which is flooded with the copper naphthenate solution. The timber is then subjected to various cycles that may involve vacuum or pressure, then the solvent is drained away and excess working solution is removed from the timber by vacuum. The treated timber is then removed from the autoclave, still wet with solvent. The residual solvent in the wood is typically left to evaporate naturally. Solvent remaining in the wood migrates to the wood surface by capillary action and eventually evaporates.
The LOSP process has a number of drawbacks. In particular, much of the solvent used is not recovered. In particular the solvent remaining in the wood after removal from the autoclave is lost to the atmosphere. The wood can also become difficult to handle and use if it is stacked in a manner which prevents complete solvent evaporation. This loss of solvent has a significant disadvantage from the points of view of cost and environmental impact. The solvents may also be hazardous, for example, be toxic, or flammable.
Further, the reliance on evaporation to remove excess solvent also means that the final drying step is slow. The result is a long entry-to-exit time of radiata pine at the treatment plant, or the release into use of incompletely dry product, leading to odour, painting and gluing problems. The delay in turnaround time to ensure adequate post-treatment drying has implications in terms of cost. Further, the LOSP process is also not amenable to the impregnation of wood with substances which have inherent low solubility in the solvent of choice. In order for wood to be retreated with an additional active agent, if this is necessary, the drying step needs to be wholly or substantially completed before the process can be repeated.
Alternative processes exist that involve contacting the wood with solvent vapours at elevated temperatures. Processes involving contacting hot solvent vapours with wood can result in the leaching of compounds, particularly fatty compounds, from the wood. This can be undesirable as the mechanical properties of the wood can be altered as a result of the leaching of compounds from the wood matrix by the hot solvent vapours.
A further alternative process uses aliphatic hydrocarbons such as propane, butane, pentane or mixtures thereof under pressure so that the aliphatic hydrocarbon is in the liquid phase as the carrier solvent. This now-discontinued Cellon (or, in Europe, Drilon) process used compressed liquid petroleum gas (LPG, typically propane and/or butane) with a co-solvent, under non-supercritical conditions, as carrier fluid for pentachlorophenol. A number of factors have been reported that tended to make the process uneconomical. These included the need to purge the treatment vessel with an inert gas before and after treatment to remove oxygen (to avoid creating explosive mixtures with the flammable LPG), the high insurance costs for such an operation, the energy required to recover the LPG (heating energy input, and long (1-3 hr) vacuum time required.
The replacement of LPG with non-flammable methylene chloride in the successor Dow process reduced the costs associated with purging and insurance, but the energy costs for recovery of this 40° C. boiling solvent were still too high. To the Applicant's knowledge, there is currently only one plant still operating with methylene chloride.
While the Cellon process has been abandoned, another process based upon a compressed gas carrier has been developed—using supercritical carbon dioxide.
A supercritical fluid is one that, under certain conditions, ceases to be a liquid and behaves like a gas, although retaining the solvent properties of a liquid. The supercritical fluid generally utilised in the wood treatment process is carbon dioxide, but other supercritical fluids can be utilised.
Carbon dioxide at a temperature greater than its critical temperature of 31.1° C. and a critical pressure of 72.9 atm. or 7.39 MPa is widely used as a supercritical fluid. This is mainly because supercritical carbon dioxide has a relatively low critical pressure and temperature, is readily available in large quantities, and is non-toxic, odourless, and non-flammable. Its surface tension becomes zero with a viscosity of about 320 pP (at 40° C. and 75.1 atm.). It is regarded as having very good solvent characteristics and is fairly environmentally sound without any exceptional pollution and waste disposal problems.
While supercritical CO2 is the least expensive supercritical fluid, in absolute terms it remains extremely expensive. The first commercial supercritical CO2 plant commenced operation in Hampen, Denmark in March 2002. The 60,000 m3yr−1 plant required very heavy engineering to cope with a design operating pressure of 15,000 kPa with correspondingly high cost. Amortisation of such high capital costs imposes considerable strain on operational economic viability. Supercritical CO2 techniques also have some other disadvantages associated with the extreme pressure changes. These can include collapse and distortion of the timber during treatment, and high variability in preservative retentions achieved. Accordingly, supercritical CO2 technology is unlikely to replace methods and apparatus which rely on more traditional solvents in the near future for general use.
Supercritical CO2 technology has however proved uniquely effective in treating refractory timbers, most particularly, spruce. Spruce is well known to be the “holy grail” of wood treatment and to date, it has only been treatable via the use of supercritical CO2 techniques. Hydrofluoroalkanes or more generically, hydrofluorocarbons (HFCs), have not been widely used in impregnation methods due to their high cost, low solvency and significant greenhouse gas potential. Hydrofluoroalkanes, are commonly used in refrigeration applications, where they have largely superseded earlier refrigerants such as anhydrous ammonia, sulphur dioxide and Freon®. Chlorofluorocarbon (CFC) and hydrochlorofluorocarbon (HCFC) refrigerants marketed as Freon® were developed in the 1920s by DuPont to replace toxic inorganic gases, rapidly gaining adoption as they were odourless, colourless, nonflammable, and noncorrosive. In the 1990s, most uses of CFCs were phased out due to their damaging effect on the earth's ozone layer, and HCFCs became prominent for a few years because they are less damaging in this regard. However, the use of HCFCs is now generally being phased out, for example, in Australia by 2012. HFCs are the predominant class of refrigerant used worldwide today, as they do not harm the ozone layer at all, although they are acknowledged as powerful greenhouse gases, some having a global warming potential (GWP) thousands of times that of carbon dioxide. For this reason, HFCs are targeted under the Kyoto Protocol and are being replaced in many domestic refrigeration applications by straight hydrocarbons (HCs), such as propane. However, HFCs are still widely and lawfully available and used with appropriate precautions and regulations against discharge to the atmosphere for many applications where their properties are not matched by HCs.
Halocarbons are named industrially after the pattern HCFC-01234a where                0=number of double bonds in the molecule (omitted if zero),        1=number of carbon atoms −1 (omitted if zero),        2=number of hydrogen atoms +1,        3=number of fluorine atoms,        4=atom replaced by bromine (“B” prefix added), and        a=a lower case suffix to distinguish isomeric forms.        
The “normal” (unsuffixed) isomer is the most symmetrical and a, b, . . . are added as molecular symmetry decreases, or A=an upper case suffix to distinguish between commercialised mixtures of refrigerants.
Table 1 shows common halocarbons and other refrigerants and their relevant physical properties.
The present applicants have surprisingly found HFC's to be highly useful impregnation solvents, especially for refractive timbers.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.
Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.